Divestiture-related activities - Schedule of Operating Results of Operations Treated as Discontinued Operations (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Dec. 31, 2016	Sep. 25, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Divestiture-Related Activities [Abstract]
Costs and other expenses									$ 4,534	$ 922	$ 1,730
Loss from discontinued operations before income taxes									(4,534)	(922)	(1,730)
Tax benefit on loss from discontinued operations									1,801	1,112	10,635
(Loss) income on discontinued operations	$ 189	$ (2,383)	$ (360)	$ (179)	$ 187	$ 122	$ 193	$ (312)	$ (2,733)	$ 190	$ 8,905

Amount before tax of income (loss) from a discontinued operation. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal.

+ References + Details

- Definition

Amount after tax of income (loss) from a discontinued operation including the portion attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

+ References + Details